Plants offer great potential as production systems for recombinant proteins. One approach to producing foreign proteins in plants is to generate stable transgenic plant lines. However this is a time consuming and labor intensive process. An alternative to transgenic plants is the use of plant virus-based expression vectors. Plant virus-based vectors allow for the rapid, high level, transient expression of proteins in plants.
One method to achieve high level transient expression of foreign proteins in plants involves the use of vectors based on RNA plant viruses, including comoviruses, such as Cowpea mosaic virus (CPMV; see, for example, WO2007/135480; WO2009/087391; US 2010/0287670, Sainsbury F. et al., 2008, Plant Physiology; 148: 121-1218; Sainsbury F. et al., 2008, Plant Biotechnology Journal; 6: 82-92; Sainsbury F. et al., 2009, Plant Biotechnology Journal; 7: 682-693; Sainsbury F. et al. 2009, Methods in Molecular Biology, Recombinant Proteins From Plants, vol. 483: 25-39).
Comoviruses are RNA viruses with a bipartite genome. The segments of the comoviral RNA genome are referred to as RNA-1 and RNA-2. RNA-1 encodes the VPg, replicase and protease proteins. The replicase is required by the virus for replication of the viral genome. The RNA-2 of the comovirus cowpea mosaic virus (CPMV) produces a polyprotein of 105 kDa or 95 kDa processed into 4 functional peptides.
The 5′ region of CPMV RNA-2 comprises start codons (AUGs) at positions 115, 161, 512 and 524. The start codons at positions 161 and 512 are in the same triplet reading frame. Initiation at the start codon at position 161 results in the synthesis of the 105K polyprotein while initiation at the start codon at position 512 directs the synthesis of the 95K polyprotein. Initiation of translation at the start codon at position 512 in CPMV is more efficient than initiation at position 161, resulting in the production of more 95K polyprotein than 105K polyprotein. The start codon at position 115 is not essential for virus replication (Wellink et al., 1993 Biochimie. 75(8):741-7).
Maintenance of the frame between the initiation sites at positions 161 and 512 in CPMV RNA-2 is required for efficient replication of RNA-2 by the RNA-1-encoded replicase (Holness et al., 1989; Virology 172, 311-320; van Bokhoven et al. 1993, Virology 195, 377-386; Rohll et al., 1993 Virology 193, 672-679; Wellink et al., 1993, Biochimie. 75(8):741-7). This requirement impacts the length of sequences which can be inserted upstream of the 512 start codon in replicative forms of CPMV RNA-2 expression vectors. Furthermore, the use of polylinkers should be used with caution as they may shift the open reading frame (ORF) between these initiation sites.
CPMV has served as the basis for the development of vector systems suitable for the production of heterologous polypeptides in plants (Liu et al., 2005 Vaccine 23, 1788-1792; Sainsbury et al., 2007 Virus Expression Vectors (Hefferon, K. ed), pp. 339-555). These systems are based on the modification of RNA-2 but differ in whether full-length or deleted versions are used. Replication of the modified RNA-2 is achieved by co-inoculation with RNA-1. Foreign proteins are fused to the C-terminus of the RNA-2-derived polyproteins. Release of the N-terminal polypeptide is mediated by the action of the 2A catalytic peptide sequence from foot-and-mouth-disease virus (Gopinath et al., 2000, Virology 267: 159-173). The resulting RNA-2 molecules are capable of spreading both within and between plants. This strategy has been used to express a number of recombinant proteins, such as the Hepatitis B core antigen (HBcAg) and Small Immune Proteins (SIPs), in cowpea plants (Mechtcheriakova et al. J. Virol. Methods 131, 10-15; 2006; Monger et al., 2006, Plant Biotechnol. J. 4, 623-631; Alamillo et al., 2006, Biotechnol. J. 1, 1103-1111). Though successful, the use of a full-length viral vector limits the size of inserted sequences, and movement between plants raises concerns about biocontainment of the virus.
To address the issue of biocontainment and insert size, Canizares et al. (2006 Plant Biotechnol, J 4:183-193) replaced the majority of the coding region of RNA-2 with a sequence of interest to produce a disabled version of CPMV RNA-2 (deIRNA-2). The sequence to be expressed was fused to the AUG at position 512 of RNA-2, immediately upstream of the 3′ untranslated region (UTR) to create a molecule that mimics RNA-2. Such constructs were capable of replication when introduced into plants in the presence of RNA-1 and a suppressor of silencing, and directed the synthesis of substantial levels of heterologous proteins (Sainsbury et al., 2008 Plant Biotechnol J 6:82-92).
Mutation of the start codon at position 161 in a CPMV RNA-2 vector (U162C; HT) increases the levels of expression of a protein encoded by a sequence inserted after the start codon at position 512. This permits the production of high levels of foreign proteins without the need for viral replication and was termed the CPMV-HT system (WO2009/087391; Sainsbury and Lomonossoff, 2008, Plant Physiol. 148, 1212-1218). In pEAQ expression plasmids (Sainsbury et al., 2009, Plant Biotechnology Journal, 7, pp 682-693; US 2010/0287670), the sequence to be expressed is positioned between the 5′UTR and the 3′ UTR. The 5′UTR in the pEAQ series carries the U162C (HT) mutation.